There are at least 13 known substances in the blood that are used to make blood. Normally, these substances are kept separate and do not come into contact with each other. However, when a blood vessel breaks, only then do they mix to produce the substances that make up the blood.
One of the various cells in the blood is called platelets. These cells contain one of the 13 substances that make blood clump together. Platelets are very fragile and are destroyed by shock when a blood vessel breaks and blood flows out. When this happens, the substances contained in the platelets are released and mingle with the other substances. As they mingle, thread-like substances form one after another, intertwine with each other, and block the broken blood vessels.

The 13 substances that make blood coagulate are called clotting factors, and if any one of the 13 is missing, the blood will not coagulate. The disease hemophilia is caused by the lack of a specific one of these 13 factors.